Process for converting silicon dioxide containing waste flue dust to crystalline zeolitic molecular sieves of type A

ABSTRACT

Silicon dioxide containing waste flue dust from the production of silicon metal and silicon alloys is converted to crystalline zeolite molecular sieves of type A by (1) dissolving the flue dust in alkali metal hydroxide solution with the formation of alkali metal silicate solution having a module (SiO 2  :Me 2   I  O) in the range of 2.0:1 to 3.5:1 at a temperature of 60°-100° C.; (2) purifying the thus obtained alkali metal silicate solution from organic constituents by treatment with activated carbon and/or an oxidizing agent and separating the non-decomposable residue from the solution; and, (3) reacting alkali metal aluminate solution present at room temperature with the diluted, purified alkali metal silicate solution with a module (SiO 2  :Na 2  O) of 2.0:1 to 3.5:1 obtained by steps (1) and (2) and subsequently crystallizing the reaction mixture ((2.0-4.4)Na 2  O:Al 2  O 3  ; (1.3-1.8)SiO 2  :(90-120)H 2  O) at a temperature of 90°-95° C. for a period of 6-12 hours, subsequently filtering, washing to pH of about 10-11 and drying to recover a crystalline, zeolitic molecular sieve of type A having a degree of whiteness according to Berger (A. Berger, Die Farbe 8 Nr. 4/6 (1959) pages 187-202) at 460μ in the range of 90-95%.

BACKGROUND OF THE INVENTION

The invention is directed to a process for the conversion and utilization of silicon dioxide containing waste flue dusts from the production of silicon metal and silicon alloys to crystalline zeolitic molecular sieves of type A.

There are known various processes for the production of alkali silicates whose aqueous solutions are employed in combination with sodium aluminate and sodium hydroxide in special precipitation methods for the recovery of zeolites:

In the melt process, which is predominantly practical today, silicon dioxide (quartz) is reacted with soda (sodium carbonate) in a melt at high temperature. This melt process has the disadvantage that there is required considerable energy input to carry it out. Besides the melt process is expensive in regard to apparatus and therefore, requires considerable capital. On account of the high operating temperature, the molten compositions, furnace ceiling and recuperators are subjected to high wear and tear. Besides the melt process is characterized by an especially careful selection of the quartz sand employed, particularly in regard to the iron and aluminum content (Ullmann's Enzyklopadie der technischen Chemie, Vol. 15, 3rd edition, pages 732-733).

In the wet decomposition process, which is also known, the reaction is difficult to carry out quantitatively since in using natural sand it is only reacted relatively slowly and incompletely. According to this wet decomposition process it is only possible to operate discontinuously in an autoclave and therewith intensive as to working and energy. The discharged waterglass is dilute and in transportation leads to increased costs (Ullmann's Enzyklopadie der technischen Chemie, Vol. 15, 3rd edition, page 736).

To partially avoid the above-mentioned disadvantages there is used as the starting material for the production of alkali silicates in Japan naturally occurring volcanic sand.

It has also been proposed already to use for the production especially pure alkali silicates, particularly for industrial purposes, pyrogenically produced, chemically high pure, amorphous silica as the starting product (Ullmann's Enzyklopadie der technischen Chemie, Vol. 15, 3rd edition, page 735). This method of production, however, has the disadvantage that because of the high cost of starting material the recovered alkali silicate solution can be employed for special purposes, however, not on a technologically broad basis.

To recover waterglass solutions from industrial waste products there has already been proposed the working up of waste silica as it precipitates in the production of Na₂ AlF₆, AlF₃, NaF and HF from the hexafluorosilicic acid, H₂ SiF₆, arising out of the raw phosphate process (Biegler, German Pat. No. 2 219 503). However, there is obtained only a waterglass with limited stability since the incompletely removed fluoride ion catalyzes the polymerization of the waterglass. Besides the content of toxic fluoride is disturbing in the subsequent working up of this material.

Recently there have been attempts to work up other high silica containing waste products to form waterglass. Thus, in the recovery of silicon carbide which is produced in the electric furnace by metallurgical process there is formed as the reaction product a large amount of wet gases which initially contain silicon monoxide which then is oxidized with the entrance of air oxygen to highly dispersed silicon dioxide. The reaction gases additionally also contain solid impurities which together with the silicon dioxide is separated in considerable amounts as flue dust from the waste gases. The resulting flue dust possesses a high content of amorphous SiO₂. As impurities besides other oxides such as Fe₂ O₃, Al₂ O₃, MgO and CaO there is also found carbon used in the form of graphite in the electrodes. It enters in finely divided form in the waste gas and gives the flue dust a black-gray color. Furthermore, the eliminated products contain impurities originating from the reaction of quartz with, for example, oil coke. Furthermore, there are also contained in the flue dust organic materials formed by thermal decomposition out of the coal tar or dextrin used as binders in the electrode material, which arrive in the SiO₂ containing waste gas as cracked products. These materials are firmly adsorbed by the extremely finely divided silica in the waste dust.

Waste flue dust also accumulates in considerable amounts in the production of silicon-iron alloys. Thus, there is formed in the reducing production of one ton of silicon-iron alloy 0.2-0.5 ton of dust. The reuse of this dust by recycling in the electric furnace to be sure has been tried but because of the necessity of a granulating process has little profitability. Since until now there has been no significant industrially or economically practical possibility of use the waste flue dust according to the state of the factory is either given off to the atmosphere as a white smoke (aerosol) and forms the basis of a severe air pollution or it is collected in the sea and rivers or is brought to deposits.

To be sure there has already been proposed the chemical preparation of silica containing waste flue dusts by decomposition with alkali hydroxide in the wet process. Thereby there is produced from the unpurified waterglass solutions obtained silica gels with the aid of acids. These silica gels, however, have only limited use because of the previously mentioned solid and soluble impurities, Japanese opened Sho No. 49-134599 and Japanese opened Sho No. 49-134593.

In the earlier unopened German patent application No. P 26 09 831.7 there is described a process for the preparation of precipitated silicas and silicates from silicon dioxide containing waste flue dusts employing the following combination of process steps:

I. Dissolution of the flue dust in alkali hydroxide solution with formation of an alkali silicate solution with high module (SiO₂ :Na₂ O);

II. Purification of this alkali silicate solution from organic constituents by treatment with activated carbon and/or an oxidizing agent and separation of the non-decomposable black residue from the solution; and,

III. Reaction of the purified alkali silicate solution with acids and/or salts of aluminum and calcium or magnesium at temperatures in the range of 60°-110° C. in the pH range of 1-12, subsequent filtration, washing and drying of the filter cake paste as well as comminution of the drier granulate to recover finely divided, amorphous, pure precipitated silica.

Although with the help of this combination of steps for the first time at present there is the possibility to convert the environment loading flue dust to an industrial use, further endeavors are necessary to increase meaningfully by chemical change the dusts whose toxicological hazards have recently been published (J. C. A. Davies, The Central African Journal of Medicine, Vol. 20 (No. 7), July 1974, pages 140-143 and D. M. Taylor, J. C. A. Davies, The Central African Journal of Medicine, Vol. 21 (No. 4), April 1975, pages 67-71).

The invention therefore is based on the problem of developing a chemical process for the preparation of crystalline zeolitic molecular sieves of type A from silicon dioxide containing waste flue dusts from the production of silicon metal and silicon alloys. Type A zeolite molecular sieve is a very valuable product with extensive possibilities for use, e.g., as an ion exchanging heterogeneous inorganic builder in modern washing agents, as adsorption agent for the drying of gases and liquids, as separating agent for the decomposition of gas mixtures, as catalysts and as catalyst constituents, as ion exchanger for drinking water and industrial waste water.

SUMMARY OF THE INVENTION

The process of the invention includes the following features:

I. The flue dust is dissolved in alkali metal hydroxide solution with the formation of alkali metal silicate solution having a module (SiO₂ :Me₂ ^(I) O) in the range of 2.0:1 to 3.5:1 at a temperature in the range of 60°-110° C.;

II. The thus obtained alkali metal silicate solution is purified from organic constituents by treatment with activated carbon and/or an oxidizing agent; and,

III. Reacting alkali metal aluminate solution present at room temperature with the diluted, purified alkali metal silicate solution having a module (SiO₂ :Me₂ IO) in the range of 2.0:1 to 3.5:1 obtained by steps I and II, and subsequently crystallizing the reaction mixture (2.0-4.4)Na₂ O:Al₂ O₃ ; (1.3-1.8)SiO₂ :(90-120)H₂ O) at a temperature of 90°-95° C. for a period of 6-12 hours, subsequently filtering, washing to pH of about 10-11 and drying to recover a crystalline zeolite molecular sieve of type A with a degree of whiteness according to BERGER (loc. cit) at 460μ in the range of 90-95%.

While mention is made of Na₂ O in the formulation above if the alkali metal of the silicate and the aluminate is potassium then the Na₂ O will be replaced by K₂ O.

The diluted alkali metal hydroxide containing mother liquor resulting from the molecular sieve crystallization in step III can be mixed with commercial 45-50% sodium hydroxide with avoidance of the energy expensive evaporation and be returned in the decomposition process of the FeSi (or Si) flue dust according to step I to alkali metal silicate solution with a module (SiO₂ :Me₂ ^(I) O) 2.0:1 to 3.5:1.

The return of the mother liquor from step III can also be carried out in such manner that solid sodium silicate with a module of SiO₂ :Na₂ O=3.3:1 dissolves in this mother liquor, the thus obtained solution undergoes a reduction in module until there is reached a module having a value of 2:1 and then is returned to step I. Thereby through decomposition of the flue dust there is likewise correspondingly controlled the amount of mother liquor to be worked up. Under these conditions there are obtained zeolite molecular sieves of type A from the thus obtained sodium silicate solutions, which in a given case undergo filtration and purification steps.

As alkali metal hydroxide solutions there can be used sodium hydroxide or potassium hydroxide.

To remove the impurities from the alkali metal silicate solutions resulting from the decomposition process there are used as oxidizing agents hydrogen peroxide or alkali metal peroxides, e.g., potassium peroxide, or more preferably sodium peroxide, which is added either during the decomposition or shortly before the separation of the undecomposed residue or also shortly thereafter.

In an especially advantageous manner the removal of the impurities by means of activated carbon treatment according to step II is carried out shortly before the separation of the non-decomposable residue of the alkali metal silicate solution or the activated carbon treatment is combined with the treatment with the oxidizing agent. As activated carbon there is particularly suited one prepared from pure wood charcoal by steam activation at red heat and having a water content of 10%, a specific surface area according to BET in the range of 600-800 m² /g, a pH of 9-10 and a fineness of grinding of 80% below 40 nm.

As proof of the industrial advance of the process of the invention below there is given a comparison of the energy requirements of the production of a zeolitic molecular sieve of type A from sand, soda and sodium aluminate by the known melt process (A) and by the process of the invention (B) from FeSi-flue dust, sodium hydroxide and sodium aluminate.

From Table 1 the numerical, experimental values are ascertained and compared to kcal/kg SiO₂ standardized values. The table contains three horizontal series of numbers: of these the first series relates to the energy requirement which is employed per kg SiO₂ in the liquid waterglass phase, while in the second series of numbers there is found the energy requirement by unit of weight (kg) of the zeolitic molecular sieve of type A (100% calculated) from the same liquid waterglass according to steps A and B. The third series of numbers finally represents the sum of the first and second series of numbers and also represents the total energy expense which is employed in order to produce crystalline, zeolitic molecular sieves of Type A by processes A and B from the SiO₂, sand source of FeSi-air dust. In the numerical comparison it is noticeable that in recovering waterglass according to melt process A the energy requirement to obtain waterglass is 3.2 times higher than in process B of the invention. While in process A the portion of energy for recovering liquid waterglass still makes up 31% of the total energy requirement, for process B according to the invention this portion is only 12% of the invention. Measurements and total energy expense show that process B of the invention in comparison to known process A brings about a 21% saving in energy.

The advance in the art of the duplex process of the invention is illustrated further by the following advantages over the known procedures.

The duplex process of the invention presents the possibility of recycling the alkali metal hydroxide containing diluted mother liquor without the use of an evaporation process as a result of which there is a considerable saving of energy. In the case of evaporation besides an expensive plant for concentrating the diluted mother liquor from 40-60 grams NaOH/l to 200 grams NaOH/l there are used 430-495 kcal/l of mother liquor which can be saved by the present invention.

In addition to silicon and ferrosilicon alloy other silicon alloys can be used in the invention, e.g., silicomanganese, chromium silicide, ferrochromium-silicon, vanadium silicide, titanium silicide and silicon-aluminum alloy.

Unless otherwise indicated all parts and percentages are by weight.

The process can comprise, consist essentially of or consist of the steps set forth with the stated materials.

The process will be further illustrated in the following examples, but they are not intended to limit the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS EXAMPLE 1

As the starting raw product there were added

(a) a flue dust resulting from the production of ferrosilicon (FeSi) with the following analytical and physical data:

    ______________________________________                                         SiO.sub.2               89.55%                                                 Fe.sub.2 O.sub.3        0.90%                                                  Al.sub.2 O.sub.3 + MgO + CaO + Na.sub.2 O + K.sub.2 O                                                  3.90%                                                  SiC                     0.40%                                                  C                       1.40%                                                  Loss on ignition (at 1000° C.)                                                                  3.20%                                                  Bulk density            250 grams/l -Particle size 60 microms                  ______________________________________                                    

(b) a flue dust resulting from the production of metallic silicon with the following data:

    ______________________________________                                         SiO.sub.2               97.5%                                                  Fe.sub.2 O.sub.3        0.10%                                                  Al.sub.2 O.sub.3 + MgO + CaO + Na.sub.2 O + K.sub.2 O                                                  2.10%                                                  SiC                     0.30%                                                  C                       1.40%                                                  Loss on ignition (at 1000° C.)                                                                  3.20%                                                  Bulk density            250 grams/l                                            Particle size           90 microns                                             ______________________________________                                    

Decomposition Process

There were filled into a 2000 liter capacity vessel of V2A-steel having a cover and equipped with a double jacket for fluid heating and with a rapid stirrer and bottom outlet valve 750 liters of water and there were dissolved 150 grams of NaOH/l therein. The mixture was heated to 70°-90° C. and there was introduced with stirring 127.5 kg of product (a) containing 89.6% SiO₂. This mixture is heated at 90°-100° C. for 40-60 minutes as a result of which the amorphous silica of the residue visibly dissolves. Then there were introduced 127.5 grams of product (b) with a SiO₂ content of 97.5%. After a further reaction time of 2-3 hours the decomposition mixture was pumped from the reaction vessel and by means of a suitable filtration apparatus (rotary filter, bond filter or filter press) was filtered off hot from carbon containing non-decomposable residue using filter cloths of Perlon. In addition it is advisable to dilute the decomposition solution before the filtration with 650 liters of hot water. For fine filtration there is used a rotary disk filter and the operation is carried out with the help of settling technology. There were obtained 1300 liters of a water white sodium silicate solution, which contained 171.2 grams of SiO₂ /l and 86.77 grams of Na₂ O/l (d=1.235) and which had a module of SiO₂ :Na₂ O=2.04:1.

Purification

To remove organic impurities there were added to the decomposition solution about 1 hour before the end of the reaction 5.0 kg of activated carbon. In order to even improve the purification effect there were added to the decomposition solution after the addition of the activated carbon sodium peroxide in an amount of 0.5 gram/l (375 grams Na₂ O₂). As an alternative procedure in place of sodium peroxide there can be added 10 ml of aqueous hydrogen peroxide solution (35%) per liter of waterglass, preferably after the filtration.

Synthesis of Molecular Sieve of Type A

In a rubberized 1000 liter container provided with a cover, double jacket for heating fluid, stirrer and bottom outlet valve there were present 420 liters of water and 200 liters of a sodium aluminate solution which contained 240 grams of Al₂ O₃ /l and 336 grams of Na₂ O/l. To this solution there were added with stirring 215 liters of the purified waterglass solution produced by decomposition (171 grams of SiO₂ /l and 87 grams of Na₂ O/l, module 2.04:1, density 1.235 g/ml). Under continuous stirring the reaction mixture (2.9Na₂ O:1.0Al₂ O₃ :1.3SiO₂ :95H₂ O) is kept at 90°-95° C. for 12 hours. There were formed zeolite crystals of type A. By subsequent filtration, washing to pH 10-11 and drying there was obtained a white powder with a degree of whiteness according to BERGER (loc. cit) at 460μ of 91.5%.

EXAMPLE 2

The production and purification of the sodium silicate solution (d=1.235 g/ml; module=2.04:1 and 171 grams of SiO₂ /l as well as 87 grams of Na₂ O/l) were carried out as in Example 1.

There were present in a 4,000 liter container 100 liters of aqueous sodium hydroxide containing 60 grams of Na₂ O/l. With stirring there were simultaneously dosed into the sodium hydroxide present at room temperature at a velocity of 21.6 liters/min 542 liters of the above waterglass solution and with a velocity of 10 liters/min 250 liters of sodium aluminate solution which contained 60 grams of Na₂ O/l and 20 grams of Al₂ O₃ /l. From the at first clear solution there precipitated out after a short time a gelatinous product. Thereto there were added under continuous stirring 1,800 liters of a hot (80° C.) sodium aluminate solution (51.8 grams of Al₂ O₃ /l and 71.1 grams of Na₂ O/l). The reaction mixture was crystallized at 93° C. for 3.5 hours. The product obtained had the X-ray structure of a zeolite of type A. The finely divided product had a degree of whiteness according to BERGER (loc. cit) at 460μ of 92%.

EXAMPLE 3

In the production of a molecular sieve of type A according to Example 1 there resulted a mother liquor which still contained part of the wash water required for the washing process. The content of NaOH amounted to 54 grams/l.

10 liters of this dilute aqueous sodium hydroxide were treated with 460 grams of solid NaOH in a container of V2A-steel having a capacity of 20 liters. After the dissolution of the solid NaOH the solution was heated to 90°-95° C. and gradually 850 grams of the FeSi-flue dust (a) were (Example 1) introduced with stirring. It was heated for 45 minutes in a closed container. Then there were added 850 grams of Si metal flue dust (b) (Example 1) and the decomposition mixture heated a further 3.5 hours at 95° C. The residue was separated off by means of a pressure filter over a Perlon cloth filter after which there were recovered 6.6 liters of a sodium silicate solution containing 203 grams of SiO₂ /l and 104 grams of Na₂ O/l, corresponding to a module of SiO₂ :Na₂ O=2.02:1. The purification of the decomposition solution was carried out according to Example 1.

After the purification, there were present in a 50 liter V2A-steel container 10 liters of water and 5 liters of a sodium aluminate solution which contained 240 grams of Al₂ O₃ /l and 335 grams of Na₂ O/l. To this solution there were added with stirring 5.5 liters of the above-purified waterglass solution produced by decomposition (203 grams of SiO₂ /l; 104 grams of Na₂ O/l, Module=2.02:1). With further stirring the reaction mixture (3.1Na₂ O:1.0Al₂ O₃ :1.6SiO₂ :92H₂ O) was held for 10 hours at 94°-96° C., the crystalline zeolite of type A formed was dried after filtration and washing to pH 10. The product obtained displayed a degree of whiteness according to BERGER (loc. cit) at 460μ of 91%.

While in the working examples there is employed a mixture of flue dusts from metallic silicon and ferrosilicon alloy it should be realized that there can be used either the flue dust from metallic silicon or the flue dust from the silicon alloy alone.

                  TABLE 1                                                          ______________________________________                                         Energy Requirement, Comparison of                                              Melt Process and Wet Process                                                   Process Variant   A          B                                                 ______________________________________                                         Energy requirement for liquid                                                  waterglass kcal/kg SiO.sub.2                                                   in liquid glass   1,310 (31%)                                                                                 412 (12.0%)                                     Energy requirement for zeolitic                                                molecular sieve of type A                                                      kcal/kg zeolitic molecular                                                     sieve of type A*  2,970 (69%)                                                                               2,970 (88.0%)                                     Total energy requirement**                                                     kcal/kg zeolitic molecular                                                     sieve of type A   4,280 (100%)                                                                              3,382 (100%)                                      ______________________________________                                          *Zeolite A calculated "atro" (active dry substance) = 100%; as spray drie      powder with 20% H.sub.2 O                                                      **Saving of energy with process B of the invention 21% compared to the         conventional process A                                                         A = Melt or vat process from sand and soda with following production of        zeolite A                                                                      B = Wet decomposition process from FeSiflue dust and sodium hydroxide wit      following production of zeolite A                                         

What is claimed is:
 1. A process for the preparation of crystalline zeolite molecular sieves of type A from waste flue dust containing silicon dioxide obtained from the production of silicon metal or ferrosilicon or is a mixture of flue dusts from the production of silicon metal and ferrosilicon comprising (1) dissolving the flue dust in alkali metal hydroxide solution with the formation of alkali metal silicate solution having the mole ratio (SiO₂ :Me₂ ^(I) O) in the range of 2.0:1 to 3.5:1 at a temperature of 60°-110° C.; (2) purifying the thus obtained alkali metal silicate solution from organic constituents by treatment with both activated carbon and sodium peroxide and separating the non-decomposable residue from the solution; and (3) reacting alkali metal aluminate solution present at room temperature with the purified alkali metal silicate solution with the mole ratio (SiO₂ :Me₂ ^(I) O) of 2.0:1 to 3.5:1 obtained by steps (1) and (2) and subsequently crystallizing the reaction mixture ((2.0-4.4)Me₂ O:Al₂ O₃ :(1.3-1.8)SiO₂ :(90-120)H₂ O) at a temperature of 90°-96° C. for a period of 6- 12 hours, subsequently filtering to remove the product from the mother liquor, washing the product to pH of about 10-11 and drying to recover a crystalline, zeolite molecular sieve of type A having a degree of whiteners according to BERGER at 460μ in the range of 90-95%, the alkali metal of the hyroxide, silicate, and aluminate being sodium or potassium.
 2. The process of claim 1 wherein the alkali metal of the hydroxide, silicate and aluminate is sodium.
 3. The process of claim 2 comprising carrying out the crystallization at a temperature of 90°-95° C.
 4. The process of claim 1 comprising mixing the mother liquor, said mother liquor containing alkali metal silicate and alkali metal hydroxide, of step (3) with more concentrated alkali metal hydroxide solution and returning the thus produced alkali metal hydroxide and alkali metal silicate solution to step (1) in order to produce an alkali metal silicate solution having the mole ratio SiO₂ :Me₂ ^(I) O of 2:1 to 3.5:1.
 5. The process of claim 4 wherein the alkali metal is sodium.
 6. The process of claim 5 wherein the more concentrated alkali metal hydroxide is 45-50% aqueous sodium hydroxide.
 7. The process of claim 1 comprising dissolving solid alkali metal silicate with the mole ratio SiO₂ :Me₂ ^(I) O=3.3:1 in the mother liquor containing alkali metal hydroxide until the solution has a mole ratio of 2:1 and then returning the resulting thus formed solution to step (1).
 8. The process of claim 7 wherein the alkali metal is sodium.
 9. The process of claim 1 wherein the activated carbon is one prepared from pure wood charcoal and has a water content of 10%, a BET surface area in the range of 600-800 m² /g, a pH of 9-10 and a degree of fineness of 80% below 40 microns.
 10. The process of claim 1 wherein the treatment with activated carbon and sodium peroxide is conducted at a point in time from shortly before the separation to a point in time shortly after said separation.
 11. The process of claim 1 wherein the treatment with activated carbon and sodium peroxide is conducted shortly before the separation of the non-decomposable residue from the alkali metal silicate solution. 